Fluorescence microscopy typically involves the use of three separate optical filters: an excitation filter used to select an appropriate wavelength from the system illumination source, an emission filter to limit the fluorescence wavelength spectrum received at a detector from an illuminated specimen, and a dichroic filter positioned between both the excitation filter and the specimen, as well as the specimen and emission filter. The dichroic filter is used to separate the excitation wavelength band from the emission wavelength band. Raman spectroscopy uses a similar combination of filters (excitation, emission and dichroic). Flow cytometry utilizes a larger number of separate emission filters for extracting specific wavelength-based information from a “flow” of cells passing through the instrumentation.
It has been recognized that the filter components of these imaging systems are an enabling factor in advancing the state of the art with respect to obtaining accurate results. In particular, hyperspectral fluorescence microscopy now provides for signal collection over many different spectral bands, producing a contiguous spectrum output. Unlike traditional spectroscopy, however, hyperspectral microscopy requires the use of a variety of different filters. Prior art techniques combine elements such as gratings and prisms with mechanically tunable filters to provide the required bandwidth characteristics. Problems with signal loss remain. The need to utilize a large number of separate filters with flow cytometry has the same concerns.
Current attempts at creating tunable optical filters for these purposes involve mechanical configurations, such as motors and MEMS devices. As such, limitations associated with, for example, movable parts, angular displacement between elements and the like result in relatively slow response times (with respect to initiating the mechanical action to implement wavelength tuning), limiting the useful applications of these mechanical configurations. Tunable filters based on acousto-optic configurations have also been developed, but are known to also be relatively slow and bulky and, moreover, are typically limited in the wavelength range across which the tuning may be performed.
Thus, a need remains in the art for a tunable optical filter that exhibits a relatively fast response time while also providing a wide tuning range as needed for a variety of different applications.